In a mail sorting system, there is a need to sort a large number of individual items into groups of such items which can be processed together. For example, mail items can be sorted into groups, with each group being intended for a particular geographical destination. For example, when handling newly received items, the sorting system may be required to sort the items on the basis of their intended final, or "outward" destinations, which may for example be relatively large geographical areas. Additionally, the items may be sorted by product group, for example the class of service required. When the system is processing mail which has been received from another sorting station, then those items may be intended for delivery within a relatively small geographical area, and the sorting system then sorts them into groups of items intended for delivery to even smaller areas, for example areas which might be covered in a single round of deliveries.
In either case, a conventional sorting system might include a number of output receptacles for receiving sorted items, with one receptacle being designated for items intended for each of the relevant geographical areas. When the number of destinations is large, this means either that the sorting system itself must be very large, including a large number of output receptacles, or that the sorting must be carried out in two or more stages, with one sorting stage being used for a relatively coarse sort, and second and any subsequent stages being used for finer sorts.
British Patent No. 1,506,516 describes a circulating apparatus including a plurality of receptacles for receiving sorted items. Received items are sorted manually and place in the appropriate receptacles, which can be emptied and replace as desired.